This invention relates to a critical liquidlevel warning circuit particularly suitable for use in a motor vehicle to indicate a low or critical level in its fuel tank. More particularly, the invention relates to a warning circuit for association with a liquid-level indicating circuit for a motor vehicle. The indicating circuit provides a voltage signal which varies between upper and lower voltage limits as a function of the liquid level sensed by a sensing device.
Related prior art includes the following U.S. Pat. Nos: 3,668,630; 3,681,753; 3,739,337; 3,740,740; 3,644,885; 3,641,544; and 3,623,043. The first four of these patents are believed to be the most pertinent.
It is an object of the invention to provide a critical liquid-level warning circuit suitable for use in a motor vehicle to provide its operator with a visual or other warning indication when the quantity of fuel remaining in the vehicle fuel tank is at a low or critical level, for example, one-eighth of full tank capacity. In the preferred form of the invention, as hereinafter described, the critical liquid-level warning circuit is capable of being packaged on the rear portion of a conventional fuel indicating gauge. This permits associative calibration of both the indicating gauge and the critical liquid-level indicating circuit. Also, the warning circuit of the invention is capable of providing a time delay of 20 or more seconds between the occurrence of a critical liquid-level and the provision of the warning signal to the vehicle operator. This time delay is desirable to prevent the occurrence of false low-fuel warnings that might otherwise occur when the vehicle is accelerating, going uphill or downhill, cornering, or undergoing operation that causes the fuel to slosh about or otherwise vary in level in a transient manner. The circuit of the invention also is capable of providing a time delay with respect to de-actuation of the warning device once it has been actuated.
The critical liquid-level warning circuit of the invention is capable of operating over an extended ambient temperature range and is provided with protection against vehicle electrical transient conditions. An important feature of the invention is that the critical liquid-level warning circuit may be calibrated during its manufacture without it being necessary to await the elapse of the time delay provided in the warning circuit. Furthermore, the warning circuit is of a design that permits integrated circuit construction, a feature highly desirable in a circuit to be produced in large volume. Also, the circuit of the invention is compatible with both magnetic gauge fuel indicating systems and thermal milliammeter gauge systems.
The invention may be better understood by reference to the detailed description which follows and to the drawing, which is a schematic electrical diagram of a liquid-level indicating circuit and of a critical liquid-level warning circuit particularly suitable for use as a vehicle low-fuel warning circuit.